In general, wind tunnels are used to study the airflow around objects or to measure aerodynamic properties of objects. As qualitative studies and quantitative measurements on airflows are very complicated in real aircraft conditions, these conditions are simulated in a wind tunnel. This means that the object of study is located in the test section of the wind tunnel in a fixed or limited position on a wind axis and the air is the element which moves around through this test section while aerodynamic effects are measured.
During testing, a thin layer of air close to the walls of the wind tunnel is decelerated compared to the main airstream velocity due to friction between the airflow through the tunnel and the wind tunnel walls. This layer of air is called the boundary layer. The amount of deceleration in the boundary layer can be expressed as a momentum loss. Boundary layers on wind tunnel walls have an influence on the wind tunnel measurements as they are not present in real conditions. Therefore, boundary layers cause a difference between wind tunnel measurements and real aircraft conditions measurements. This is why boundary layer effect in wind tunnels should be minimized as much as possible.
In the state of the art the following solutions for removing a boundary layer of a wind tunnel are known.
Locating a suction system of a porous or perforated section in the ground plane upstream of the model, where a certain portion of the boundary layer flow is removed by the action of pressure pumps which are in communication with the boundary layer at the wind tunnel wall and which are connected to suction device(s) by means of one or more suction ducts. The air sucked out of the wind tunnel by the suction system can be re-injected in the wind tunnel behind the measurement section.
It is also known an apparatus for sucking of near-wall air boundary layer from a flow which is led to a wall moved in parallel with the flow and delimiting the flow, which is formed by an endless belt guided around a roll, wherein in the return area of roll a suction device is arranged. The apparatus consists of a suction box provided with suction channel and of a suction plate which is provided with passage openings running perpendicular to the direction of flow and is arranged at the level of the moved wall, wherein the suction plate is laterally connected with the suction box. The suction plate bridges the space between the roll and front side of the suction box, that front edge of the suction plate is rounded and that, seen in radial direction of the roll, a distance of about 5 mm is maintained between the rounded front edge of the suction plate and the circumference of the roll.